Optimization of Spherical Head Forming Process

GUIDE:

At present, large head forgings are prone to problems such as uneven wall thickness, local thinning, and wrinkles. In order to overcome the above problems, this paper proposes the optimization of head forging process.

Through Deform-3D numerical simulation analysis, compared with the head punching process.

The forming slabs and punching aids are optimized to avoid local thinning and wrinkling of the head, and the optimized head forming process finally meets the requirements.

The head is an important component of chemical, nuclear power and other equipment.

In order to ensure that the equipment operates efficiently and at high temperature and pressure for a long time, the performance requirements are also increasing.

The head obtained by integral forging has higher strength, and has greater resistance under high temperature and high pressure hydrogen, as well as has a wide application prospect.

The integrity of nuclear power heads will directly affect the safety and life of nuclear reactors.

The head is subjected to high temperatures and pressures during the work process, and special service conditions also impose stricter requirements on the materials used in nuclear reactor pressure vessels:

(1)Appropriate strength and toughness at room temperature and working temperature and the lowest brittle transition temperature possible;

(2)Good weldability and hot and cold workability;

(3)It has the greatest tissue stability at the working temperature;

(4)It has sufficient hardenability and uniformity of thick cross-section texture.

There are two kinds of common heads: ellipsoidal head and spherical head.

In this article we mainly introduce the spherical head punch forming, and optimizes the head forming aids by numerical simulation to ensure the uniform wall thickness of the spherical head after punching.

Brief Introduction of Spherical Head Forging Process

Cutting the bottom, feed head and tongs hold with ingot→ Upsetting and compression→ Drawing a long cutting→ Upsetting and compression→ Upsetting process→ Heat treatment after forging slab→ Slab Roughing→ Slab Inspection→ Slab bending.

Deform model establishment

The three-dimensional solid model uses the three-dimensional modeling software UG to establish a three-dimensional solid model of spherical head forming upper mold and finishing. As shown in Figure 1, Figure 2.

Spherical head forming upper die

Figure 1 Spherical head forming upper die

Finished drawing of spherical head

Figure 2 Finished drawing of spherical head

During the forming process of the spherical head, the forming upper die is connected with the movable cross beam of the hydraulic press, the forming lower die is placed on four corner posts, and the punching stroke H = 1300mm.

After the punching is completed, the movable cross beam of the hydraulic press is lifted, and the finished head is lifted by a crane.

Select SA508-3 when making the material model. This kind of  steel has excellent process stability and weldability and high strength.

This material data is not available in the Deform material library. You need to get the real stress-strain curve of the material according to the material properties experiment.

As shown in Figure 3.

SA508-3 material stress-strain curve

Figure 3 SA508-3 material stress-strain curve

During the slab forming process, the forming slab is defined as a deformed body, and the forming die is defined as a rigid body.

The friction between the forming slab and the mold is a very complicated physical phenomenon, which is related to various factors of the contact surface. Such as the relative hardness, surface roughness, temperature, normal stress and relative sliding speed between contact surfaces, the advantages also change during the deformation process.

There are two types of shear friction and Coulomb friction in Deform.

This article chooses the type of shear friction. The direct friction coefficient between the deformed body and the upper mold is defined as μ = 0.4, and the friction coefficient between the deformed body and the lower mold is defined as μ = 0.3. The punching temperature was set to 1000 ° C.

Numerical Simulation Analysis of Spherical Head Forming

According to the size of the finished drawing, a slab drawing is designed. A series of numerical simulations are used to optimize the slab and mold.

The size of the slab before forming is verified by the simulation results to determine the forming slab. As shown in Figure 4.

Forming slab size

Figure 4 Forming slab size

After punching (stroke 1300mm), the comparison between the spherical head forging and the fine drawing is shown in Figure 5. The numerical simulation of the head forgings obtained according to this scheme can meet the finishing requirements.

The remaining margin of the bottom of the spherical head is about 10mm on one side, and the open end of the spherical head is about 25mm on one side.

It can be seen from the figure that the spherical head forging meets the size requirements for finishing.

Comparison of spherical head forgings and refined drawings

Figure 5 Comparison of spherical head forgings and refined drawings

The equivalent stress after forming is shown in Figure 6. The maximum deformation at the open end of the spherical head is equivalent to the closing process and caused a concentration of stress, about 40 MPa, the minimum deformation at the bottom of the spherical head, and the minimum forming stress.

The equivalent strain after forming is shown in Figure 7. The iso effect becomes 0.02 to 0.2 mm / mm. The forming force is shown in Figure 8. The maximum forming force is about 2600t.

It can be seen from the numerical simulation results of the spherical head punching that the bottom thinning is more serious.

You have to adjust the forming angle of the lower die, optimize the arc at the bottom of the upper die, and finally ensure that the head margin is uniform after punching.

Equivalent stress diagram of spherical head after punching

Figure 6 Equivalent stress diagram of spherical head after punching

Equivalent strain diagram of spherical head after punching

Fig. 7 Equivalent strain diagram of spherical head after punching

Forming force of spherical head punch

Figure 8 Forming force of spherical head punch

Optimization and Numerical Simulation of Spherical Head Punching Die

First, you need to analyze the impact of the punching die angle on the head slab forming effect, and then adjust the forming die angle to compare the forming force.

There is no obvious difference between the forgings and the finishing allowances after the simulation through comparison.

However, considering that the head diameter of this regulator is smaller and the length of the straight section is larger, the larger the forming angle of the punching die, the smaller the punching force.

The better the guiding effect when punching the slab, the less likely it is that the slab will wrinkle.

On the premise that the forming temperature is 1000 ° C and the shape of the slab is unchanged, the comparison of forming forces is shown in Figure 9.

Effect of die angle on forming force

Figure 9 Effect of die angle on forming force

The larger the mold angle, the smaller the forming force.

When the lower die angle is 16 °, the forming force is 2600t; when the lower die angle is 21 °, the forming force is 2400t; when the lower die angle is 35 °, the forming force is 2000t.

By comparison, it can be found that the larger the angle, the smaller the force required to form the slab.

Through the comparison of the forming forces, the final forming angle of the spherical head punching die was determined to be 35 °.

Modify the forming slab, increase the thickness of the bottom of the spherical head, and increase the bottom by 25mm to ensure that the wall thickness of the spherical head slab after punching is uniform, and increase the margin of the bottom of the spherical head. The modified slab is shown in Figure 10.

Optimized forming slab

Figure 10 Optimized forming slab

After optimizing the spherical head slab and punching aids, the spherical head punching results are shown in Figure 11.

Spherical head punching

Figure 11 Spherical head punching

The equivalent stress at the open end of the spherical head is the largest, about 40MPa.

The comparison between the spherical head forging after punching and finishing is shown in Figure 12.

Comparison of spherical head forging and finishing

Figure 12 Comparison of spherical head forging and finishing

Spherical head forgings have uniform margins in all parts, the inner wall of the forgings is in good contact with the punch, and the margins are uniform.

The margin on the bottom of the spherical head is about 20mm, the margin on both sides of the open end of the spherical head is about 20mm, which meets the requirements. The forged spherical head forging is shown in Figure 13.

Spherical head forging

Figure 13 Spherical head forging

Conclusions

(1) When the spherical head is designed to form a slab, the wall thickness at the risk point should be increased. The design angle of the punching die should be between 30 ° and 40 ° to avoid local thinning of the head due to the increase in forming force. Through a series of numerical simulations, the slab and forming aids are continuously optimized to ensure that the head is punched and formed at one time. The complete forging streamline is retained to provide a good structural basis for subsequent heat treatment.

(2) By adjusting the gap between the punch and the die, the fillet of the die is optimized to avoid the spherical head from being damaged during punching. You need to control the speed and stroke of the hydraulic press. The hydraulic press should use first-level pressure when starting punching to avoid excessive speed during punching.

(3) After the punch contacts the head slab, and when the punching stroke reaches H = 600 – 700mm, the forging end point is set so that the internal stress of the head slab is fully released during the punching process. If wrinkling and folding are found during actual punching, it should be stopped in time. After the slab is returned to the furnace for heating, punching is performed to prevent the head slab from getting stuck on the lower die and causing the bottom of the head to become thin.

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